One more defect and they could not have existed.
(George Louis Leclerc, Comte de Buffon)

Hence we conceive of the individual animal as a small world, existing
for its own sake, by its own means. Every creature is its own reason to
be. All its parts have a direct effect on one another, a relationship
to one another, thereby constantly renewing the circle of life; thus we
are justified in considering every animal physiologically perfect.
(Johann Wolfgang von Goethe)

We are losing animals. I do not mean only numerically through
the extinction of species. I also mean we are losing them in our
understanding. Perhaps it might be better to say we've rarely taken
animals as whole, integrated beings seriously and therefore they have
never really come into view for us. For that reason our scientific
and technological culture can so casually manipulate what it does not
know. The more we get to know something intimately, the less
likely we are to treat it in a purely utilitarian fashion.

Imagine a biotechnologist wondering what causes the sloth to be slow
and pondering whether the animal could be mined for "slothful"
genes that might be put to therapeutic use in hyperactive children. Or
another who wonders whether the sloth might not be a good research model
for testing the efficacy of genes from other organisms that enhance
metabolic activity. As far as I know, no such research projects are
in progress or being planned — and I am glad. But how easily we can
come up with ideas that hover in splendid isolation above any deeper
concern for the animal itself! We are intrigued and motivated by the
seemingly boundless limits of doing the doable. We do not feel limited
by ignorance of what we're dealing with.

This essay is an attempt to show how we can take steps to overcome some
of that ignorance — of which we should nonetheless always be mindful
— by
beginning to grasp something of the organic lawfulness inherent in one
animal, the sloth. With all its unique and unusual features, the sloth
almost seemed to be prodding me to understand it in an integrated,
holistic way. The poet and scientist Goethe set the stage for a sound
holistic approach to studying animals, and others have developed his
method further.1
I have been influenced and inspired by their work in this study.

The Sloth in its World

Even if you were to look hard and make lots of noise, you would most
likely not see the most prevalent tree-dwelling mammal in Central and
South America's rain forests. The monkeys scurry off and perhaps
scream. The sloth remains still and hidden.

The rain forest is a tropical ecosystem characterized by constancy of
conditions. The length of day and night during the year varies little. On
the equator there are twelve hours of daylight and twelve hours of night
365 days a year. The sun rises at 6 am and sets at 6 pm. Afternoon rains
fall daily throughout most of the year. The air is humid (over 90%)
and warm. The temperature varies little in the course of the year,
averaging 25°C (77° F).

Except in the uppermost part of the forest canopy, it is dark in the
rain forest. Little light penetrates to the forest floor. The uniformity
of light, warmth and moisture — in intensity and rhythm —
mark the rain forest. And it is hard to imagine a rain forest dweller
that embodies this quality of constancy more than the sloth. From
meters below, the sloth is sometimes described as looking like a clump
of decomposing leaves or a lichen-covered bough. The sloth's hair is
long and shaggy, yet strangely soft. The fur is brown to tan and quite
variable in its mottled pattern. Especially during the wettest times
of year, the sloth is tinted green from the algae that thrive on its
pelage, which soaks up water like a sponge (Aiello 1985).

Since the sloth moves very slowly and makes few noises, it blends into
the crowns of the rain forest trees. It took researchers many years to
discover that up to 700 sloths may inhabit one square kilometer of rain
forest (Sunquist 1986). Only 70 howler monkeys inhabit the same area.

The sloth spends essentially its whole life in the trees. It hangs
from branches by means of its long, sturdy claws, or sits nestled in
the forks of tree branches. The contrast to terrestrial mammals in
respect to orientation is emphasized by its fur. Instead of having a
part on the mid-back, with the hair running towards the belly, as is
typical for terrestrial mammals, the sloth's fur has a part on the
mid-belly and the hair runs toward the back.

The sloth moves slowly through the forest canopy — from a few to
rarely a few hundred feet in twenty-four hours. On average, sloths were
found to move during seven to ten hours of the twenty-four-hour day
(Sunquist and Montgomery 1973). The remaining time sloths are asleep
or inactive. (Resting is the term often used to describe the sloth's
inactive periods, but this isn't a sloth-appropriate expression. From
what activity is the sloth resting?)

Limbs and Muscles

The sloth's ability to hang from and cling to branches for hours on
end is related to its whole anatomy and physiology. The sloth is about
the size of a large domestic cat. It has very long limbs, especially
the forelimbs (Figure 1). When hanging, the sloth's body appears to
be almost an appendage to the limbs. Feet and toes are hidden in the
fur. Only the long, curved and pointed claws emerge from the fur. The
toe bones are not separately movable, being bound together by ligaments,
so that the claws form one functional whole, best described as a hook.

Figure 1. The three-toed sloth. (Sketch by Craig Holdrege.)

The two different genera of sloths are named according to the number
of claws they possess: the three-toed sloth (Bradypus) has three
claws on each limb; the two-toed sloth (Choloepus) has two claws on
the forelimb and three on the hind limb. (There are many differences
in detail between these two groups of sloths. Most of the specific
information referred to in this essay pertains to the three-toed sloth,
unless otherwise indicated.)

With its long limbs the sloth can embrace a thick branch or trunk,
while the claws dig into the bark. But the sloth can also hang just by
its claws on smaller branches, its body suspended in the air. A sloth
can cling so tenaciously to a branch that researchers resort to sawing
off the branch to bring the creature down from the trees.

All body movements, or the holding of a given posture, are made
possible by muscles, which are rooted in the bones. Muscles work by
means of contraction. While clinging, for example, some muscles in the
limbs — the retractor muscles — are contracted (think of
your biceps) while other muscles — the extensor muscles —
are relaxed (think of your triceps). When a limb is extended (when the
sloth reaches out to a branch) the extensor muscles contract, while the
retractor muscles relax. All movement involves a rhythmical interplay
between retractor and extensor muscles.

It is revealing that most of a sloth's skeletal musculature is made up
of retractor muscles (Goffart 1971; Mendel 1985a). These are the muscles
of the extremities that allow an animal to hold and cling to things
and also to pull things toward it. The extensor muscles are smaller and
fewer in number. In fact, significant extensor muscles in other mammals
are modified in the sloth and serve as retractor muscles. A sloth can
thus hold its hanging body for long periods. It can even clasp
a vertical trunk with only the hind limbs and lean over backward ninety
degrees with freed forelimbs. As the sloth expert M. Goffart points out,
"in humans this feat is exceptional enough to be shown in a circus"
(Goffart 1971, p.75).

At home as it is in the trees, the sloth is virtually helpless on
the ground. Lacking necessary extensor muscles and stability in its
joints, a sloth on the ground can hardly support its weight with its
limbs. Researchers know little about natural terrestrial movement of
sloths. But on rough surfaces captive sloths have been observed slowly
crawling (Mendel 1985b). If they are placed on a smooth surface like
concrete, their limbs splay to the side. In this position a sloth can
only drag its body by finding a hold with the claws of its forelimbs
and pulling itself forward, using its strong retractor muscles.

Since the sloth's main limb movements involve pulling and the
limbs do not carry the body weight, it is truly a four-armed and not a
four-legged mammal. The hands and feet are essentially a continuation of
the long limb bones, ending in the elongated claws and do not develop
as independent, agile organs as they do, say, in monkeys. We can also
understand the dominance of the retractor muscles from this point of
view. The human being, in contrast to most mammals, has arms as well
as weight-bearing legs. The arms are dominated by retractor muscles,
while the legs have more extensor muscles. Moreover, the arm muscles
that move the arm toward the body are stronger than the antagonistic
arm muscles that move the arms away from the body. This comparison
shows us that the tendency inherent in the arm — the limb that does
not carry the body's weight — permeates the anatomy of the sloth.

A sloth becomes quite agile if the forces of gravity are reduced, as in
water. In water a body loses as much weight as the weight of the volume
of water it displaces (Archimedes' Law). The body becomes buoyant,
and in the case of the sloth, virtually weightless.

Remarkably, sloths are facile swimmers . . . . They manage to move across
water with little apparent effort. Where the forest canopy is interrupted
by a river or lake, sloths often paddle to new feeding grounds. With no
heavy mass to weigh them down, they float on their buoyant, oversized
stomachs. (Sunquist 1986, p. 9)

With its long forelimbs the sloth pulls its way through the water,
not speedily, but in a "beautifully easy going" manner (Bullock,
quoted in Goffart 1971, p. 94).

On the whole, sloths have little muscle tissue. Muscles make up 40
to 45 percent of the total body weight of most mammals, but only 25
to 30 percent in sloths (Goffart 1971, p. 25). One can understand how
the reduction of weight in water allows them to be less encumbered in
movement. Sloth muscles also react sluggishly, the fastest muscles
contracting four to six times more slowly than comparable ones in a
cat. In contrast, however, a sloth can keep its muscles contracted six
times longer than a rabbit (Goffart 1971, p. 69). Such anatomical and
physiological details reflect the sloth's whole way of being —
steadfastly clinging in a given position, only gradually changing
its state.

The tendency to the reduction of muscle tissue can also be found
in the head. There is a reduction in the number and complexity of
facial muscles (Naples 1985). Through the facial markings the sloth
has an expressive face, but this is the expression of a fixed image,
rather than expression through movement, since the facial area itself
is relatively immobile. The outer ears are tiny and are essentially
stationary. The sloth alters the direction of its gaze by moving its
head, not its eyeballs. This rather fixed countenance is dissolved at
the lips and nostrils, which, as the primary gateways to perceiving
and taking in food, are quite mobile.

Paced Metabolism and Fluctuating Body Temperature

Since sloths are externally inactive or asleep a good portion of the
twenty-four-hour day and the remaining time is spent slowly moving and
feeding, they perform about ten percent of the physiological work of a
mammal of similar size (Goffart 1971, p. 59). All metabolic processes
are markedly measured in tempo. Sloths use little oxygen, breathe slowly,
and the respiratory surface of their lungs is small.

All metabolic activity produces warmth. Warmth is also needed for
activity, for example, in the exertion of muscles, which in turn results
in more warmth production. Birds and virtually all mammals not only
produce warmth, but also maintain a constant body temperature. This is
a striking physiological feat. A warm-blooded (endothermic) animal is
like a radiating, self-regulating center of warmth. Warmth constantly
permeates the whole organism.

Most mammals maintain a constant core body temperature of about 36°C
(97°F), which changes very little despite variations in environmental
temperatures. For example, in a laboratory experiment a mouse's
internal temperature changes only four tenths of one degree Celsius
when the outer temperature rises or falls twelve degrees (Bourlière
1964). Exceptionally, however, a sloth's body is not so permeated by
warmth; in other words, it is not constantly prepared for activity. Its
body temperature can vary markedly.

Gene Montgomery and Mel Sunquist, who did extensive field research
in Panama on the ecology and physiology of sloths, found that the
sloth's body temperature fluctuated with the ambient temperature
(Montgomery and Sunquist 1978). During the morning as the ambient
temperature rose, the body temperature also rose. When found on sunny days,
sloths were often on an outer branch, belly-side up with limbs extended,
basking in the sun. Body temperature usually peaked at about 36-38°C
soon after midday. It then began to fall, reaching a low point of
about 30-32°C in the early morning. The body temperature was usually
about 7-10°C higher than the ambient temperature.

Although sloths are often active at night, their body temperature does
not rise in connection with their increased activity. This shows, in
contrast to other mammals, that the sloth's body temperature is less
affected by its own activity than by the ambient temperature. According
to Brian McNab (1978), the sloth "almost appears to regulate its
rate of metabolism by varying body temperature, whereas most endotherms
[warm-blooded animals — mammals and birds] regulate body temperature
by varying the rate of metabolism." Raising the outer temperature
under experimental conditions is, as Goffart puts it, an efficient
way of "'deslothing' the sloth," since it then moves around
more actively. But if its temperature remains at 40°C for too long,
it can prove fatal.

A three-toed sloth has an insulating coat of fur comparable to that of
an arctic mammal, which seems at first rather absurd for a tropical
animal. It has, like an arctic fox, an outer coat of longer, thick
hair and an inner coat of short, fine, downy fur. These allow the
sloth to retain the little warmth it creates through its metabolic
processes. But, characteristically, the sloth cannot actively raise its
body temperature by shivering as other mammals do. Shivering involves
rapid muscle contractions that produce warmth.

Clearly, the sloth is at home in the womb of the rain forest, which
keeps constant conditions like no other terrestrial ecosystem. Not
only by virtue of its coloring and inconspicuous movements does the
sloth blend into its environment, but through its slowly changing body
temperature as well.

Feeding and Orientation

Moving unhurried through the crown of a tree, the sloth feeds on
foliage. We usually think of leaf eating (browsing) as an activity
done on the ground by mammals, for example, deer. There are, in fact,
relatively few leaf-eating mammals in the crowns of trees, although
tree leaves are an abundant and constant source of food. Sloths are
literally embedded in and surrounded by their food at all times and in
all directions. Tropical trees do lose their leaves, but not all at
once. Sometimes one and the same tree may lose leaves on one branch,
while it sprouts new ones on others.

Sloths don't eat just any leaves. They seem to prefer younger leaves,
and each individual animal has its own particular repertoire of about
40 tree species from which it feeds (Montgomery and Sunquist 1978). A
young sloth feeds together with its mother, often licking leaf fragments
from the mother's lips. After its mother departs the juvenile at the
age of about six months, the young sloth continues to feed from those
species it learned from its mother. This specificity is probably a major
factor in the inability to keep three-toed sloths alive in zoos. They
usually die of starvation after a short period of time. In contrast,
the two-toed sloth is more flexible and survives well in captivity,
eating assorted fruits and leaves.

A sloth does not search for leaves with its eyes. Its eyesight is
very poor and it is short-sighted (Goffart 1971, pp. 106ff.; Mendel
et al. 1985). The eyes lack the tiny muscles that change the form of
the lens to accommodate for changing distances of objects. As if to
emphasize the unimportance of its eyes, the sloth can retract them into
the eye sockets. The pupils are usually tiny, even at night. Clearly,
a sloth does not actively penetrate its broader environment with its
vision, as do most arboreal mammals like monkeys.

Sight and hearing (the latter also not very developed in sloths) are
the two senses through which animals perceive and react to stimuli at
a distance. The sloth makes little use of these senses, relying much
more on a sense that entails drawing the environment into itself:
the sense of smell.

I placed a sloth, hungry and not too disturbed, on an open area under
the bamboos, and planted four shoots twenty feet away in the four
directions of the compass. One of these was Cecropia [a primary food
of three-toed sloths] camouflaged with thin cheesecloth, so that the
best of eyesight would never identify it, and placed to the south,
so that any direct wind from the east would not bring the odor too
easily. The sloth lifted itself and looked blinkingly around. The bamboos
thirty feet above, silhouetted against the sky, caught its eye, and it
pitifully stretched up an arm, as a child will reach for the moon. It
then sniffed with outstretched head and neck, and painfully began its
hooking progress toward the Cecropia .... Not only is each food leaf
tested with the nostrils, but each branch.... (Beebe 1926, p. 23)

So we should not imagine a sloth looking at its food. Rather, a sloth
immerses and orients itself in a sea of wafting scents.

When the sloth is in the immediate proximity of leaves it feeds on, it
will hook the branch with the claws of a fore- or hind limb and bring
the leaves to its mouth. Having no front teeth (incisors), it tears
off the leaves with its tough lips. It chews the leaves with its rear,
peg-like teeth. Unlike most leaf-eating mammals (for example, deer),
the sloth lacks many deeply rooted, hard, enamel-covered grinding
teeth. The sloth also has comparatively few teeth (18 compared to
32 in most deer). Moreover, the teeth lack enamel altogether and
wear easily. In compensation, the teeth grow slowly throughout the
animal's life. There is no change of teeth from milk to permanent
dentition. Growth and wear are in balance.

While feeding, the sloth is continuously chewing and simultaneously
moving food backward with its large tongue in order to swallow. Sloths
can feed in all positions, even hanging upside down. A young, captive
two-toed sloth showed "decided preference for eating upside down in
the manner of adult sloths at eight months" (Goffart 1971, p. 114).

The sloth can move its head in all directions, having an extremely
flexible neck. Imagine a sloth hanging from all four legs on a horizontal
branch. In this position the head looks upward (like when we lie in
a hammock). Now the sloth can turn its head — without moving
the body — 180 degrees to the side and have its face oriented
downwards. As if this were not enough, the sloth can then move its head
vertically and face forward — an upright head on an upside down
body (Figure 2)! When it sleeps, a sloth can rest its head on its chest.

Figure 2. The three-toed sloth. Note the orientation of the head. (Sketch
by Craig Holdrege.)

The sloth's neck is not only unique in its flexibility, but also in its
anatomy. Mammals have seven neck (cervical) vertebrae. The long-necked
giraffe and the seemingly neckless dolphin — to mention the extremes
— both have seven cervical vertebrae. This fixed mammalian pattern
is abandoned by only the sloth and the manatee. The three-toed sloth
usually has nine and the two-toed sloth has between six and nine
cervical vertebrae.

Centered in its Stomach

Digestion in the sloth occurs at an incredibly slow rate. In captive
animals "after three or six days of fasting the stomach is found
to be only slightly less full" (Britton 1941). Leaves are hard to
digest and not very nutrient-rich, consisting primarily of cellulose
and water. Only with the help of microorganisms in the stomach can the
sloth digest cellulose, breaking it down into substances (fatty acids)
that can be taken up by the blood stream.

The sloth's stomach is four-chambered like those of ruminants (cows,
deer, and so on) and is clearly the center of the digestive process. The
stomach is enormous relative to the animal's overall size. It takes
up most of the space of the abdominal cavity and, including contents,
makes up 20 to 30 percent of total body weight. Nonetheless, digestion
takes a long time. On the basis of field experiments, Montgomery and
Sunquist (1978) estimate that it takes food about ten times longer
to pass through a sloth than through a cow. Moreover, the sloth also
digests less of the plant material than most other herbivores.

Through its stomach a mammal senses hunger. Most grazing mammals spend
a large part of their time eating, so that food is continuously passing
through their digestive tract. The sloth is, once again, an atypical
herbivore since it feeds for a comparatively small portion of its day. A
small rain forest deer, the same size as a sloth, ate six times as much
during the same period of time (Beebe 1926). The howler monkey, which
also lives in the canopies of Central and South American rain forests
and whose diet comprises only about 50% leaves, eats about seven times
as many leaves as do sloths. With its slow metabolism and digestion,
the sloth's stomach remains full, although the animal eats so little.

As a stark contrast, we can think of carnivores like wolves or
lions that regularly, as a normal part of their lives, experience
empty stomachs. Their hunting drives are directly connected to their
hunger. Hunger brings about the maximal aggressive activity of these
animals. When a lion has gorged itself on forty pounds of meat,
it becomes lethargic and sleeps for an extended period. The
sloth's constantly full stomach is a fitting image for its consistently
slow-paced life as well as, it seems, a physiological condition for it:
"starvation makes [sloths] hyperactive" (Goffart 1971 p. 113).

After about a week of feeding, sleeping and external inactivity, a
change occurs in the sloth's life. It begins to descend from its
tree. Having reached the forest floor, it makes a hole in the leaf
litter with its stubby little tail. It then urinates and defecates,
covers the hole, and ascends back into the canopy, leaving its natural
fertilizer behind. (The two-toed sloth has no tail and leaves its feces
lying on the leaf litter.)

The feces, the product of sloth metabolism, decompose very slowly. The
hard pellets can be found only slightly decomposed six months after
defecation. Normally, organic material decomposes rapidly in the warm
and moist conditions of the rain forest. For example, leaves decompose
within one or two months (a process that can take a few years in a
temperate-climate forest). Ecologically, sloth excrement "stands out as
a long-term, stable source [of nutrients] ... and may be related to
stabilizing some components of the forest system .... Sloths slow the
normally high recycling rates for certain trees..." (Montgomery and
Sunquist 1975, p. 94). Sloths contribute not only slow movement to the
rain forest but slow decomposition as well!

It is estimated that a sloth can lose up to two pounds while defecating
and urinating, more than one fourth of its total body weight (Goffart
1971, p. 124). If one imagines a sloth with a full stomach (which it
always seems to have) just prior to excreting, then more than half of
its body weight is made up of its food, waste and digestive organs! This
quantitative consideration points to the qualitative center of gravity
in the animal's life. But the sloth's stomach is more like a vessel
that needs to remain full than a place of intensive muscular activity,
secretion, mixing and breaking down, as it is in the cow, for example.

Stretching Time

The sloth researcher William Beebe wrote in 1926: "Sloths have no
right to be living on this earth, but they would be fitting inhabitants
of Mars, whose year is over six hundred days long." Beebe was deeply
impressed by the way in which sloths "stretch" time, another way of
characterizing their slowness. We have seen how this quality permeates
every fiber of their day-to-day existence. It is therefore not so
surprising to find that the development of sloths takes a long time.

Sloths have a gestation period of four to six months, compared to a
little over two months in the similar-sized cat. One two-toed sloth
in a zoo gave birth after eight-and-one-half months. Initially more
surprising was the rediscovery of a female sloth in the rain forest
15 years after she had been tagged as an adult. This means she was at
least 17 years old, "an unusually long life span for such a small
mammal" (Montgomery, quoted in Sunquist 1986). Thus, regarding time,
the qualities of the sloth certainly speak a unified language.

Gravity and the Skeleton

If we look for the embodiment of fixed form in the organ systems of
a mammal, then we come to the skeleton. The bony skeleton gives the
mammal its basic form and is the solid anchor for all movement. The limb
bones develop their final form in relation to both gravity and their
own usage. An injured quadruped mammal will lose bone substance in
the leg it is not using, which does not carry any weight. Conversely,
in the other three limbs bone matter is laid down to compensate for
the increase in weight carried and muscular stress.

The sloth has a special relation to gravity. As mentioned earlier,
the limbs hold the hanging body; they do not carry it (Figure 3). The
sloth gives itself over to gravity rather than resisting it and living
actively within it via its skeletal system. A sloth kept on the ground
in a box developed raw feet from the unaccustomed pressure (Beebe 1926).

Figure 3. Skeleton of a three-toed sloth. (Reprinted from Young 1973, 600.)

Figure 4. Skeleton of a horse. (Reprinted from Tank 1984, 108.)

The other pole in relation to gravity is represented by hoofed mammals
like deer, horses or giraffes. By virtue of their skeletal architecture
they can relax their muscles and even sleep while standing. Their
legs are solid, stable columns that carry the body's weight (Figure
4). In contrast, the sloth has very loose limb joints. In his detailed
study of the limbs of the two-toed sloth, Frank Mendel (1985a, p.159)
points out how unusual the "poorly reinforced and extremely lax joint
capsules" are. This anatomical peculiarity allows a wide range of
limb movement and is connected with the fact that the joints are not
subject to compression as they are in weight-bearing limbs. Through
clinging and hanging, the joints of a sloth are being continually
stretched. Similarly, the sloth has a flexible, curved spine. The hoofed
mammal, in contrast, has a stiff, straight spine, from which the rib
cage and internal organs of the torso are suspended. A deer would be
as ungainly in a tree as a sloth is on the ground.

This contrast is mirrored in the teeth. Hoofed mammals have deeply
rooted, very hard teeth with ridges of enamel that withstand
the toughness of grass. Enamel is the hardest substance a mammal
can produce, and, as already mentioned, sloth teeth have no enamel
coating. In addition, more than in other mammals, the form and
chewing surfaces of the sloths' teeth are sculptured during usage.
"Since sloth teeth acquire their individual characteristics through
wear, it is very difficult to distinguish the young of one genus from
those of another based upon shape or location of dentition" (Naples
1982 p. 18). In other mammals — especially the grazers —
the teeth are preformed with all their crown cusps and ridges before
they erupt. The sloth's teeth emerge as simple cones and take on a
characteristic form in the course of life.

The sloth is, in this sense, formed from the outside. In a related way
we see this tendency in its coloring, which arises not only from hair
pigmentation but also through algae from the surroundings. Similarly,
its temperature varies with the ambient temperature.

From a different vantage point we can say: incorporating solidity and
stability into the skeleton allows a quadruped mammal to live actively
within gravitational forces. In giving itself over to gravity, the sloth
incorporates inertia. We see inertia in its movements and digestion. The
sloth is a bit like the clump of leaves or the alga-covered tree trunk
it outwardly resembles.

Drawing In

Active arboreal mammals, like monkeys, have, of course, nothing of the
skeletal rigidity of ground-dwelling quadrupeds. They have flexible
joints and muscular agility that allow for actively swinging, jumping,
and grasping. A sloth lacks the quick and nimble dexterity of monkeys,
although it possesses a flexible spinal column (especially in the
neck region) and limber fore- and hind limbs. A sloth can twist its
forelimb in all directions and roll itself into a ball by flexing its
vertebral column.

Characteristically, the sloth makes use of this flexibility for its slow
movements while feeding and also for protecting itself from a predator
by curling up into a ball. The monkey, in contrast, engages in light
and springy movements. This leads us to a slightly different way of
characterizing the sloth. A primary gesture is that of pulling in or
retracting. It doesn't project actively out into its surroundings.

We can see this tendency in the head. The head is the center of
the primary sense organs through which an animal relates to its
environment. As we have seen, the eyes and ears are not the sloth's
main senses. The outer ears (pinnae) are tiny and hardly visible on
the head and the eyes can retract in their sockets. Both of these
characteristics reveal externally the muted function of these organs
within the whole animal. They also let the head appear as a broadened
neck. But this appearance also has a deeper anatomical basis, since the
first cervical vertebra (the so-called atlas) is nearly as wide as the
widest part of the skull.

The skull itself is rounded and self-contained — superficially
resembling a monkey's skull more than a grazing herbivore's
(Figure 5). Most herbivores have an elongated snout that they use as
a limb — standing as they do on all four legs — to reach
their food. The sloth's forelimbs have this function and thus its
snout is short. The premaxillary bones — important in forming
the elongate mammalian snout — are tiny in the sloth. Moreover,
the upper jawbones (maxillae) and the nasal bones are also short in
the sloth. The sloth's skull does not project forward.

Figure 5. Skulls of a three-toed sloth (top, left), new world monkey
(top, right), and horse (bottom). (Drawings by Craig Holdrege; the
skulls are not drawn to scale.)

We have seen that the sense of smell is the sloth's primary sense
and that its gesture is to draw in, in contrast to the more outwardly
projecting senses of sight and hearing. When we see these facts together
with the others, such as the dominance of retractor muscles, then the
sloth's special orientation to its surroundings comes more clearly
into view.

The Sloth as a Habitat

As if to emphasize its passive, somewhat withdrawn character, the sloth
functions as a habitat for myriad organisms. I have mentioned the algae
that live in its fur, giving the pelage a greenish tinge. In addition to
the usual ticks and flies that infest the skin and fur of other mammals,
a number of sloth-specific moth, beetle, and mite species live on the
sloth and are dependent upon it for their development. The sloth moths
and beetles live as adults in the sloth's fur. Some species live on
the surface and others inhabit the deeper regions of the fur. They are
evidently not parasitic; their source of food is unknown.

When the sloth descends from a tree to defecate and urinate, female
moths and beetles fly off the animal and lay their eggs in the sloth's
dung. The wings of one moth species break off soon after they inhabit
the sloth, so that they are incapable of flying. Consequently they must
crawl off the sloth to reach the dung. The sloth's relatively long
period of defecation, which lasts a few minutes, gives the insects the
time they need. In this way the slowness of the sloth serves these most
"slothful" of sloth moths!

The larvae develop in and feed on the dung (which, you remember,
decomposes slowly). The larvae pupate in the dung and the winged adult
moths (or beetles) fly off to inhabit another sloth. Various species of
insects and mites inhabit any given sloth, and the numbers of specimens
of each species varies greatly, ranging from a few to over a hundred.

The sloth has been observed grooming its fur. This is typical mammalian
behavior and does rid an animal of some of its "pests." From
this utilitarian point of view, the sloth's grooming is not very
effective. Typically, sloths groom slowly, and sloth moths "may be
seen to advance in a wave in front of the moving claws of the forefoot,
disturbed, but by no means dislodged from the host" (Waage and Best
1985 p. 308). Clearly, the measured pace of life, the unique excretory
habits, and the consistency of dung allow the sloth to be a unique
habitat for such a variety of organisms.

Sensing a Boundary

The expression of pain is a barometer for the way an animal experiences
its own body in relation to the environment. Pain is one way an animal
experiences the external world penetrating and harming its biological
integrity. Here's an example from a family that kept a sloth at their
home in Brazil:

'Sloth burning!' . . . we leap to our feet and run frantically round
trying to discover where [the sloth] has fallen asleep. On the kitchen
stove? No! On the water heater in the bathroom? No! There he is on top
of the floor lamp in the drawing-room, with his bottom touching the big
electric bulb!.... We struggle to get him down, but he clings desperately
to his perch, refusing to budge and protesting with many ah-eees against
our unwarranted disturbance of his slumbers. (Tirler 1966, p. 27)

Sloths are reported to "survive injuries that would be deadly within
a short time to other mammals" (Grzimek 1975). "I have known a
sloth to act normally for a long time after it had received a wound
which practically destroyed the heart..." (Beebe 1926 p. 32). These
examples show that the sloth does not seem to notice an intrusion of
its boundaries and continues to live despite them. Its body is not
imbued with sensitive reactive presence.

A Further Dimension of Wholeness: The
Environment?

Where does the sloth end? This seemingly naïve question points
to a problem and, at the same time, to a task. The problem is the way we
think of an organism in relation to its environment. The environment is
that with which an animal interacts. Inasmuch as the sloth eats leaves,
leaves belong to its environment. In the moment it is interacted with
(for example, in feeding, smelling, moving), the environment is
part of the animal. We could also say, the animal is part of its
environment. The environment as a functional concept is inseparable from
the organism (Riegner 1993). The corpus of an animal with its definite
outline — what we call the body — fills a definable volume
in space. But the animal's activity carries beyond this corpus. And
the environment is part of this activity; without the environment there
would be no activity.

It may seem strange to say that the environment is not outside the
animal. But this is only because we use spatial terms to describe
something functional. Because it is more natural for us to think
about the world in the framework of objects, we consider the organism
here and the environment there. But this accounts only for
the bodily aspect of the organism, and not its functional and behavioral
relations. When we shift our focus from the body as a thing to the body
as focal point of activity, then the organism encompasses, firstly,
all activities radiating to and from this focal point and, secondly,
everything we consider to be outside the organism before we change
to the functional mode of viewing — leaves, branches, scents, and
so on. (I have spoken and will continue to speak of organisms and
their environments, otherwise I would have to create some new, probably
cumbersome, terminology. There is no problem using existing terminology
as long as we can see through it to the expanded concept.)

Viewed in this way, organisms actually interpenetrate. Sloth, tree,
sloth moth, and algae are all part of each other. We can, therefore,
in principle, understand how an ecological community, an ecosystem,
and even the whole earth can be considered as further dimensions of
organisms. Speaking of the earth as an organism is then no longer merely
an analogy, but becomes a reality one has in part begun to grasp — in
this case, through the sloth. (And because we have already seen a part,
we have also caught a glimpse, in it, of the whole!) It remains the task
of a truly holistic or organismic ecology to concretely apply this way
of viewing to ecological phenomena.

Is There a Cause of Slothfulness?

In his compendium on sloths (1971), M. Goffart includes one section
entitled "Slothfulness." He describes observations in the field,
experimental results, and the hypotheses of scientists concerning the
causes of slothfulness. Various possible explanations are brought forth:
small heart, slow speed of muscle contraction, low body temperature, low
rate of thyroid function, and so forth. He describes the shortcomings
of each particular hypothesis and concludes that the "evidence
as to the real causes of slothfulness is thus far from complete"
(p. 95). If he were writing today, he might include conjectures about
genetic mechanisms.

Goffart points out, for example, that the sluggish koala has a constant
body temperature of 36 degrees Celsius. Since this is a normal body
temperature for mammals, it seems evident that it cannot be causing
the koala's sluggishness. Since causes are assumed to be general, he
concludes that temperature will also not be the cause of slothfulness
in sloths.

Goffart assumes that the causes of slothfulness will one day be
found; we are just lacking the necessary information. I question this
assumption and believe that such an example shows, in fact, primarily
the limitations of the conceptual framework. In treating aspects of
an organism as potential causes, we conceptually lift them out of the
organism. Then we think of them affecting things in the organism as
though they were not part of it. By so doing we can think in general
terms of the factor "body temperature" as a cause, as if separate
from the organism.

But every time we carry through this process we realize that our
conceptual scheme doesn't fit reality, because we are confronted
with mutual relations, all of which express something of the animal as
a whole. If we drop this scheme, then it becomes interesting that body
temperature evidently means two very different things in the koala and
the sloth. Instead of looking for genetic or physiological causes that
we assume have general validity, we look at the unique expression of
physiological facts in the given context. We take the unique integrity
of each animal seriously.

It is second nature for a scientist to inquire after the causes of
what is under investigation. Some would even say this is the task
of science. But in the context of organisms this method alone is
not adequate. Putting it a bit radically, biologists would do well
to eradicate the term "cause" from their vocabulary and use the
more modest and open term "condition." What genetic, physiological,
behavioral, and ecological studies can show is how aspects of an organism
provide mutual and changing conditions for each other. This knowledge is
extremely valuable as long as we don't separate it from the organism
as a whole. In fact, it can be the gateway to understanding the organism
as an integrated whole.

Encircling the Unspeakable: The Animal as a Whole

I'd like to return to the statements quoted at the beginning of
this essay: George Louis Leclerc, Comte de Buffon was a well-known
18th-century French scientist. He studied many animals, among them
the sloth, about which he said: "one more defect and they could not
have existed" (quoted in Beebe 1926). He considered the sloth's
remarkable characteristics to be defects. And they are, if you take
the point of view of a horse, eagle, jaguar, or human being. But as
naturalist William Beebe countered, "a sloth in Paris would doubtless
fulfill the prophecy of the French scientist, but on the other hand,
Buffon clinging upside down to a branch of a tree in the jungle would
expire even sooner" (Beebe 1926 p. 13).

Buffon takes a standpoint outside the animal. I have followed Goethe's
suggestion and tried to view the sloth on its own turf. He wrote:

Hence we conceive of the individual animal as a small world, existing
for its own sake, by its own means. Every creature is its own reason to
be. All its parts have a direct effect on one another, a relationship to
one another, thereby constantly renewing the circle of life; thus we are
justified in considering every animal physiologically perfect. (Goethe
1995, p. 121)

I have made use of comparison, but not to describe what the sloth
"should" have in order to be a reasonable animal. The animals
described by way of comparison shed light on the sloth, allowing its
uniqueness to stand out all the more perceptibly. When Goethe calls an
animal "perfect," he means that each animal has its own unique way
of being — its specific integrity that we can try to understand. But
this is no simple matter. Goethe recognized that "to express the
being of a thing is a fruitless undertaking. We perceive effects and a
complete natural history of these effects at best encircles the being of
a thing. We labor in vein to describe a person's character, but when
we draw together actions and deeds, a picture of character will emerge"
(1995, p. 121; translation modified by CH). In trying to paint a picture
of the sloth, I have discussed many details, because through them the
whole lights up. Henri Bortoft puts it well when he says, "The way
to the whole is into and through the parts. The whole is nowhere to be
encountered except in the midst of the parts" (1996, p. 12).

This emergent picture of the whole does not and cannot encompass the
totality of its characteristics. One can always discover new details. I
am not striving for totality, but rather for wholeness. Our understanding
hinges on our ability to overcome the isolation of separate facts and to
begin to fathom the animal as a whole, integrated organism. The whole
is elusive, and yet, at every moment, potentially standing before the
mind's eye. When we begin to see how all the facets of the animal
are related to each other, then it comes alive for us. Or, putting it
a bit differently, the animal begins to express something of its life
in us. Every detail can begin to speak "sloth," not as a name,
but as a qualitative concept that a definition can do little justice to.

I have tried to describe the sloth in a way that allows us to catch
glimpses of its wholeness. I can now refer to such characteristics
as slowness, inertia, blending in with the environment, receding or
pulling in and not actively projecting outward. Each expression is a
different way of pointing to the same coherent whole. Taken alone, as
abstract concepts or definitions, they are empty. They are real only
inasmuch as they light up within the description or perception of the
animal's characteristics. But they are not things like a bone or an
eye. They are, in context, vibrant concepts that reveal the animal's
unique way of being.

Let's return to the sloth, high in the crown of a rain-forest tree,
hanging from or nestled on a branch. In its outer aspect, it blends
in with its environment. There are no sudden or loud movements. The
sloth's green-tinged, mottled brown coat lets it optically recede
into the wood and foliage of its surroundings. And like the tree bark,
the sloth's fur is teeming with insect life. The sloth's body
temperature rises and sinks with the ambient temperature.

The round form of its head is the anatomical image of the way in which
the sloth does not actively project into its environment. There are no
large, movable, reactive outer ears. and the eyes are rarely, if ever,
moved. The sloth has no protruding snout. It draws the scents of the
environment, especially of the leaves it feeds upon, into its nose. But
much of the day the sloth is curled up, unaware of the world around
it. Even when awake, the sloth seems not to live as intensely in its
body as other mammals, being quite insensitive to pain.

The sloth does not carry its own weight; rather, it clings to an outer
support. Its skeletal system is not characterized by stability, but by
looseness. This laxity allows the sloth to adopt positions that would
be contortions in other animals. The sloth makes mostly steady pulling
movements with its long limbs, a capacity based on the dominance of
retractor muscles.

The sloth develops slowly in the womb and has a long, slow life. It
moves slowly through the crowns, feeding on the leaves that surround it
from all sides, bathing, as it were, in its food source. The leaves pass
through the animal at an almost imperceptibly slow rate. The sloth's
stomach is always filled with partially digested leaves. Even its dung
disappears slowly, despite the warm and humid rain forest climate that
normally accelerates decomposition processes.

The sloth brings slowness into the world.

Copyright 1998, 2009 The Nature Institute. This is a revised version of
an article originally published in the Newsletter of the Society for
the Evolution of Science vol. 14, no. 1 (1998), pp. 1-26.

Riegner, Mark (1993). "Toward a Holistic Understanding of Place:
Reading a Landscape through Its Flora and Fauna," in Dwelling, Seeing
and Designing: Toward a Phenomenological Ecology, edited by David
Seamon. Albany NY: SUNY Press.

Riegner, Mark (1998). "Horns, Hooves, Spots, and Stripes: Form and
Pattern in Mammals," in Seamon and Zajonc 1998, pp. 177-212.